Molded plastic tanks for holding and processing wastewater, have been long used in substitution of traditional concrete tanks. Plastic septic tanks that are commercially available are generally cylindrical and may have rectangular or round cross section. They have been made by blow molding or rotational molding. A typical tank is made of polyethylene having a thickness of one-quarter to three-eights of an inch. Tanks are often heavily ribbed or corrugated for strength, to resist unique forces when buried in soil and used. Typical volume capacities for domestic systems are in the range 1000-1500 gallons. Septic tanks must have access ports with closures, to permit maintenance. Most septic tanks have at least one baffle which limits lengthwise flow within the tank and divides the tank into an input end portion and the output end portion. Tanks also must resist the weight of overlying soils and possible vehicles or other weights which may be placed on the soil surface.
Advantages of molded plastic septic tanks include light weight and better resistance to impact damage and corrosion, and improved resistance to damage in the event of freezing. Disadvantages include light weight (which makes them buoyant), and limited structural rigidity which can result in distortion during installation and use. Septic tanks are periodically emptied as part of routine maintenance, to remove accumulated sludge. In the past few years, certain regulators have required that tanks pass certain tests which measure the propensity for distortion during installation or use. In one kind of test, a tank is subjected to a partial vacuum, e.g., a pressure 2 to 7 inch Hg less than standard atmospheric pressure. Such vacuum tends to pull the tank inwardly, simulating it is thought the pressure on an emptied tank which is buried in the soil, particularly in wet or non-cohesive soil.
Prior art plastic septic tanks have commonly had corrugations and other features for resisting the foregoing kinds of loads. In making tanks, the weight and cost of plastic material are major considerations. Therefore, one design objective is to maximize the volume to surface area ratio. Another consideration is to limit the size of excavation which is required. One approach is to have an oblong cross section tanks, e.g., where the cross section approximates a rectangle or square. However, such shape can present relatively large lightly curved or nearly flat surfaces. Tanks of such shape can need more strengthening than does a nearly circular cross section tank.
Tanks which have generally circumferential corrugations, with any cross section, can be susceptible to contracting lengthwise when subjected to interior vacuum or to soil forces which apply similar forces. When a tank having circumferential running ribs contracts, that behavior has been called “accordioning” by some. If tanks shrink in length when installed, or otherwise deform, and the soil in response moves inwardly, there can be undesirable diminution in the nominal volume of the tank. Another factor which affects tanks is that, when one is subject to heating by the sun or atmosphere prior to installation, that can lower the strength and stiffness of the plastic material, and make it more susceptible to distortion during handling or initial installation. So more strengthening may be needed for tanks than would appear from an analysis of the forces which result when they are buried.
Certain obvious ways of strengthening are nor particularly desirable because they tend to increase cost quite a bit. As a simple example, a tank may be made of a thicker material, or stronger material type. Thus there is a continued need for improvement in septic tanks and other like tanks for storing liquids when buried in soil and the like.